Cellular and molecular mechanisms of impaired dopaminergic action during aging. A major focus of the past year has been the elucidation of the mechanism(s) by which striatal D2 dopamine receptor-containing neurons die during aging, and the relationship of decreased receptor gene expression to this process. Primary cultures of neonatal rat striatal neurons have been utilized to examined dopamine induced cell killing, which is observed at concentrations as low as 50muM. In addition much of this effect appears to be due to apoptosis based on dual labeling of D2 receptor neurons and DNA fragmentation. In contrast to the situation for in vivo loss of D2 receptor neurons, D1 receptor neurons do not exhibit decreased expression of this receptor gene during aging. Also, in contrast to our previous findings in cultured neonatal cells, D1 receptor neurons in vivo actually appear to be more vulnerable to kainic acid killing than those containing D2 receptors. Striatal G-protein linked signal transduction during aging. We have partially characterized an assay to directly measure the coupling/uncoupling of the Gq/11 alpha subunit from the receptor ternary complex using monoclonal antibodies specific for this protein. Interaction is sensitive to both GTP analogs and muscarinic agonists in solubilized striatal membrane preparations. In addition, preliminary results suggest that the effect of muscarinic agonists is blunted in preparations from aged rats, consistent with our hypothesis of impaired G protein coupling/uncoupling. The ternary complex, as well as its individual components can also be visualized on sucrose density gradients to better follow the G protein cycling process. We have also observed an age-related decrease in striatal membrane bilayer width and an increase in cholesterol to phospholipid ratios, further consistent with our hypothesis that G protein cycling changes are secondary to membrane compositional changes during advanced age.